The invention relates to a computed tomography apparatus which includes a radiation source which emits a conical radiation beam, which apparatus involves a helical relative motion between the radiation source and the examination zone. The invention also relates to a computer program for such a computed tomography apparatus.
A computed tomography apparatus of this kind is known already from WO 9936885 (PHQ 98020). The helix and the detector unit which detects the conical radiation beam to the other side of an examination zone are proportioned in such a manner that the detector unit detects all rays of the radiation beam which extend through two neighboring segments of the helix which face the radiation source or extend between these segments. The reconstruction utilizes only the measuring data from the detector unit which is associated with the rays in the measuring window thus defined.
Upon its entry into the conical radiation beam, an arbitrary point in the examination zone is irradiated from a direction which is 180° opposed to the direction wherefrom it is irradiated upon its departure from the radiation beam. Because each point in the examination zone is irradiated only through an angular range of exactly 180°, this method is very susceptible to scanning errors.
This susceptibility is reduced in a computed tomography apparatus which is known from U.S. application Ser. No. 09/368,850 (PHD 98086); the computed tomography apparatus disclosed therein is distinct from the previously mentioned known computed tomography apparatus in that the measuring data used for the reconstruction is situated within a measuring window which is a factor of 2n+1 larger in relation to the distance between two neighboring turns of the helix, n being an integer number amounting to at least 1. According to this method each point of the examination zone is irradiated from an angular range of (2n+1). 180°. The susceptibility to scanning errors is then less pronounced. This advantage is achieved at the expense of the fact that the object to be examined is irradiated by a factor of 2n+1 longer (with the same speed of rotation). In the case of a moving object, this fact may give rise to motional unsharpness. Moreover, it may give rise to specific artefacts in the reconstructed CT image.